A photo-acid generator is a generic name of compounds which are decomposed by irradiation with an active energy ray, such as light, an electron beam, or an X-ray, to generate an acid, and the acid generated through active energy ray irradiation is used as an active species for various reactions, such as polymerization, crosslinking, and a deprotection reaction. Specific examples of such a reaction include polymerization of a cationically polymerizable compound, a crosslinking reaction of such a compound with a phenol resin in the presence of a crosslinking agent as well as an acid-catalyzed deprotection reaction of a polymer prepared by introducing a protective group to an alkali-soluble resin.
In recent years, while production of electronic parts and semiconductor element formation are performed actively by making full use of a photolithographic technique, chemically amplified resists using acids which are generated from photo-acid generators are used widely. Mass production of semiconductor elements has been carried out using a KrF excimer laser or an ArF excimer laser as an exposure light source, and shorter wavelength light sources such as an electron beam, an EUV (extreme ultraviolet ray), and an X-ray have also been investigated.
Triphenylsulfonium salts are known to have high light sensitivity to the above-mentioned exposure light sources and are used commonly as photo-acid generators for chemically amplified resists.
However, since triphenylsulfonium salts are high in crystallinity because of their symmetrical cation structures, they exhibit low solubility in solvents such as propylene glycol monomethyl ether acetate (PGMEA) and ethyl lactate, for example. Accordingly, they have practical problems, such as a limited amount of addition and incapability of being dispersed uniformly in a composition.
In order to increase the solubility in a solvent, there have been proposed a triphenylsulfonium salt in which the para position of the phenyl ring has been substituted with a fluorine atom, a fluorine-substituted alkyl group, or the like (Patent Document 1, 2) and a triphenylsulfonium salt in which the para position of the phenyl ring has been substituted with an alkyl group (Patent Document 3). However, it is known among those skilled in the art that if a substituent is introduced to the para position of a triphenylsulfonium salt, the light sensitivity is lowered as compared with the unsubstituted salt although the solubility in a solvent is increased.
Although a triphenylsulfonium salt in which the meta position of the phenyl ring has been substituted with an alkyl group (Patent Document 4) has been proposed, the research done by the present inventor has shown that the introduction of an alkyl group to the meta position of a triphenylsulfonium salt lowers the light sensitivity as compared with the unsubstituted salt although the solubility in a solvent is increased.